In image-forming devices, such as copy machines and printers, which use the electrophotographic method, images are generally formed as follows. First, toner having a positive or negative charge is electrostatically affixed to an electrostatic latent image formed on a photoconductive member (photoreceptor), so as to form a toner image. Then, this toner image is transferred to and fixed on a transfer material such as transfer paper.
Toners used for this kind of image formation generally have an average particle diameter of 5 .mu.m to 20 .mu.m, and generally include at least a colorant and a binder resin for fixing the colorant, etc. to the transfer material (transfer paper etc.).
In the past, various toners have been used as developing agents for developing latent images formed on photoreceptors in electrophotographic image-forming devices. One conventional method of manufacturing toner is, for example, grinding. This is a manufacturing method in which materials such as colorant, charge control agent, and anti-offset agent (mold release agent) are melted and kneaded together with a thermoplastic resin.
This mixture is then cooled and hardened, and then ground and separated to produce toner particles.
Another method is suspension polymerization, in which materials such as charge control agent are mixed and dispersed with polymerizable monomers, polymerization initiator, colorant, etc. This mixture is then polymerized in water. Again, there are wet methods such as the suspension granulation method, in which a colorant and a charge control agent are added to a synthetic resin. This mixture is then melted, suspended in a non-solvent medium, and granulated.
However, with toner produced by these manufacturing methods, the charge control agent, anti-offset agent, etc. exists within the toner particles. Further, only a small amount of these additives exists on the surface of the toner particles. For this reason, the charging quantity of the toner shows a wide distribution, and accordingly there are problems with toner scattering and image fogging. There are also cases when sufficient anti-offset effect cannot be obtained.
Further, the charging quantity of the toner is generally controlled by a friction charging member such as a carrier or a charging blade. If the charging quantity is more than the optimum quantity, image density is too low, but if it is less than the optimum quantity, fogging, toner scattering, etc. occur, leading to deterioration of image quality.
In order to prevent these kinds of problems, a charge control agent is generally internally added to the toner. For example, charge control agents added to positive-charging toners include nigrosine-based dyes, pyridinium salt, ammonium salt, and lake compounds of these.
However, although these charge control agents internally added to the toner are fine particles, they have a wide particle size distribution, and have no set shape. Accordingly, control of the state of their dispersal within the particles of binder resin is difficult. For example, if the particles of charge control agent dispersed within the binder resin particles are too large in diameter, the charge control agent is likely to separate out during successive copying, dirtying the charging member (carrier etc.). Again, if the particles of charge control agent dispersed within the binder resin particles are too small in diameter, their charge controlling effect is weakened. This has the drawback that the supplied toner has a slow charging response, giving rise to image fogging, toner scattering, etc.
Further, the proportion of internally added charge control agent which is exposed on the surface of the toner particles differs according to the dispersal conditions at the time of production. Accordingly, another drawback is that the charging quantity of the toner is difficult to stabilize. In addition, it is even more difficult to control the dispersal of the charge control agent with toners formed by polymerization.
As discussed above, it is difficult to take full advantage of the effects of charge control agents, anti-offset agents, etc. if they are merely internally added to the toner.
An alternative method of controlling toner charging is a technique for applying mechanical impact force, using a particle surface modification device, to attach to the surface of the toner particles chargeable inorganic particles made of a material such as silica, alumina, or titanium oxide, which have been surface processed with a material such as silane coupler or silicon oil.
However, in order to give the toner sufficient chargeability using these chargeable inorganic fine particles, they must be used in great quantity. Again, in order to fully attach the chargeable inorganic fine particles to the surface of the toner particles by means of mechanical impact force, attachment processing must be continued until surface unevenness of the toner particles is eliminated, even when non-spherical toner particles are used. As a result, toner particles which have undergone attachment processing become perfect spheres without points, which impairs blade cleaning and leads to poor cleaning.
In recent years, the development of high-speed copy machines, environment-responsive copy machines, etc., has created a need for development of toner capable of low-energy fixing low-temperature fixing). Accordingly, as a means of attaining low-temperature fixing, methods using toners including binder resins with low glass transition points or softening points have been investigated.
One example of a technique for attaining low-temperature fixing is Japanese Examined Patent Publication No. 36586/1982 (Tokukosho 57-36586), which discloses a toner which uses as binder resin a crystalline polymer having a melting point of 50.degree. C. to 150.degree. C. and an activation energy of 35 kcal/mol or less.
Further, Japanese Unexamined Patent Publication No. 87032/1975 (Tokukaisho 50-87032) (corresponding to U.S. Pat. No. 3,967,962) discloses a toner which uses a polymer formed by chemical bonding of a crystalline polymer with a melting point of 45.degree. C. to 150.degree. C. and a non-crystalline polymer with a glass transition point of 0.degree. C. or lower.
Again, Japanese Unexamined Patent Publication No. 3446/1984 (Tokukaisho 59-3446) (U.S. Pat. No. 4,528,857) discloses a toner which uses a block co-polymer, in which a crystalline block, with a melting point of 50.degree. C. to 70.degree. C., is included in a non-crystalline block molecule with a glass transition point 10.degree. C. higher than the melting point of the crystalline block.
However, use of these conventional low-temperature-fixing toners was difficult because of such problems as toner filming phenomenon caused by the soft portion of polymers, deterioration of toner chargeability, photoreceptor characteristics, etc. in, for example, successive copying, and blocking phenomenon. In other words, attaining low-temperature fixing by using toners with low glass transition points or softening points had serious problems such as deterioration of the toner's resistance to blocking, not to mention filming phenomenon and offset phenomenon.
For this reason, in the past, a method of adding an external additive to prevent deterioration of the toner's resistance to blocking has been adopted.
However, if this external additive is not attached to the toner particles but can move freely, it moves from the toner to the carrier when the carrier and toner are mixed, thus changing the quantity of charging, etc. As a result, the toner's stability over time (toner life during successive copying) deteriorates, which leads to impairment of image quality.
In recent years, the electrophotographic process has been adopted in various fields such as printers, facsimiles, color copy machines, and high-speed copy machines, and thus toners are needed which combine various characteristics (such as control of charge polarity) corresponding to these various fields and functions.
In response to this need, numerous electrophotographic toners of a type called "surface-modified toner," which gives the electrophotographic toner various characteristics, are being investigated. Some examples of surface-modified toners are a toner to which are added fine particles having various functions, such as charge control agent; an electrophotographic toner in which durability, fixing characteristics, etc. are improved by using fine particles of hardened resin to cover the surface of core particles having a low softening point; and a toner which improves charging characteristics, fluidity characteristics, etc. by means of processing to make the toner particles spherical.
In particular, many surface-modified toners have been proposed in which surface-modifying fine particles of, for example, charge control agent are dispersed over and attached to the surface of core particles of colorant, and then affixed or formed into a film thereon. For example, Japanese Examined Patent Publication No. 17576/1989 (Tokukohei 1-17576) discloses an electrophotographic graphic toner in which particles of colored resin powder are covered with a layer of a fine powder of resin or polymeric material having a particle diameter of not more than 1/10 of that of the colored resin powder. This toner is formed by coverage processing until the particles of fine powder are embedded over part of the surface of each particle of colored resin, and then heating to fuse the particles of fine powder together, forming a covering on each particle of colored resin.
Again, Japanese Unexamined Patent Publication No. 3171/1992 (Tokukaihei 4-3171) (corresponding to U.S. Pat. No. 5,206,109) discloses a manufacturing method in which surface-modifying fine particles are attached to the surface of core particles, uniformly affixed thereto by application of mechanical impact force, and then uniformly fixed or turned into a film thereon by heating in a hot air flow at 200.degree. C. to 600.degree. C.
Again, Japanese Examined Patent Publication No. 56502/1993 (Tokukohei 5-56502) proposes a surface-modified toner in which mechanical impact force is applied to attach fine powder having various functions, 2 .mu.m or less in average particle diameter, to the surface of particles of a binder resin powder made chiefly of binder resin. In this toner, attachment is performed by imbedding the particles of fine powder in the surface of each particle of binder resin powder, so that the thickness of the surface layer produced will be 2 .mu.m or less, while heating at a temperature of at least 48.degree. C., but below the melting point of the binder resin.
Japanese Unexamined Patent Publication No. 34971/1993 (Tokukaihei 5-34971) discloses the following method of manufacturing electrophotographic toner. First, in a processing room, a rotating member is rotated, mixing toner core particles (chiefly made of at least resin) with surface-processing fine particles in a high-speed air flow. By means of this mixing, the fine particles can be uniformly dispersed and attached over the surface of each toner core particle. Then, by intensifying the mixing conditions, the fine particles attached to the surface of the toner core particles are fixed and/or turned into a film thereon.
However, electrophotographic toners produced by the grinding or wet methods discussed above, which are not surface-modified toners, have the following problems.
Generally, electrophotographic toners have charging characteristics (including polarity) which vary according to the needs of the object for which and the environment in which they are to be used. In other words, different types of electrophotographic toner include different quantities of charge control agent, etc. Accordingly, when a single electrophotographic toner manufacturing device is to be used to manufacture different types of electrophotographic toner, any previously manufactured toner remaining in the manufacturing device will cause problems such as increase of the quantity of toner with reverse polarity in the subsequently manufactured toner, decrease of the toner's charging stability, etc. In order to avoid these problems, different production lines are usually provided for toners with different polarity, or thorough maintenance cleaning of the manufacturing device is performed.
Again, even with electrophotographic toners of the same polarity, according to the required characteristics, different types of charge control agents are used. The composition of toners also varies. Accordingly, even when manufacturing electrophotographic toners of the same polarity, if the same manufacturing device is to be used, thorough maintenance cleaning of the manufacturing device must be carried out, as above, in order to avoid contamination from different charge control agents or toner materials.
In this way, when manufacturing electrophotographic toners of this type, maintenance cleaning of the manufacturing device must be performed whenever the type of toner is changed. This accordingly has drawbacks such as increase in the cost of manufacturing the toner arising from the costs of cleaning and of materials discarded and wasted at the time of cleaning.
In addition, the foregoing conventional methods of manufacturing surface-modified toners merely propose methods of affixing/forming a film of surface-modifying fine particles on the surface of core particles by mechanical or heat processing, or toners produced by such methods.
These conventional manufacturing methods perform mechanical impact or heat processing to obtain a toner with long life, in which the surface-modifying fine particles on the surface of the core particles will withstand the stress of use without peeling or separation. Accordingly, the toner particles produced are made spherical, which reduces friction with the cleaning device, leading to problems such as poor cleaning.
Further, the actual state of the toner obtained by surface modification is determined only by visual means such as observing the surface of particles of surface-modified toner through an SEM (Scanning Electron Microscope). In other words, the state of the toner is not grasped quantitatively, either during or after manufacturing. Accordingly, with the conventional manufacturing methods, it is difficult to determine whether the surface-modified toner which has been manufactured sufficiently realizes desired functions. As a result, there is a great possibility that a toner will be manufactured which is not uniform and which lacks stability.
In addition, none of the conventional art gives any consideration to the weight-average molecular weight of the polymer particles (surface-modifying particles) to be affixed or made into a film on the surface of the core particles.
Incidentally, there is a method of evaluating the state of surface-modified toner produced which uses the BET specific surface area, based on N.sub.2 adsorption. The BET specific surface area of surface-modifying particles to be affixed to the surface of core particles is specified in Japanese Unexamined Patent Publication No. 335357/1992 (Tokukaihei 4-335357). However, the BET specific surface area of the surface-modified toner produced is not discussed. Further, this disclosure does not hit upon the idea of quantitatively grasping the state of surface modification.
If the core particles for surface-modified toner are to be manufactured by polymerization, facilities for control of dangerous substances such as monomers and initiators, processing of waste water, etc. are necessary, which requires large investments in facilities and increases the expenses of repayment of these investments. Further, washing and drying processes take a long time, thus reducing productivity. In addition, since the fine powder cannot be reused, manufacturing costs are increased in comparison with grinding.
In addition, since in this case the electrophotographic toner particles obtained are nearly spherical, their reduced friction results in reduced attaching force. Spherical toner particles also have a negative effect on the cleaning process. This cleaning process is the removal, using a cleaning brush, etc., of untransferred toner remaining on the photoreceptor after transfer of the toner image. When the untransferred toner is spherical, it has insufficient attaching force with respect to the cleaning brush, and its removal is made difficult.
Further, in actual use of electrophotographic toner in, for example, a high-speed copy machine (copy speed of 60 sheets/minute or more), there are cases when high stress may be applied within the developer, etc. At this time, this stress may cause peeling or separation of the fine particles of charge control agent from the surface of the core particles, leading to so-called image fogging. Accordingly, in such cases, stronger affixing/film film formation of the fine particles of charge control agent on the surface of the core particles is needed.
However, this kind of stronger affixing/film formation cannot be realized with manufacturing methods in which all processing is carried out in a surface modification device such as a Henschel-type mixer. For this reason, electrophotographic toner which is to be used in a device which applies high stress thereto should preferably be manufactured using a high-energy-applying surface modification device capable of affixing/film formation by applying high shearing force, high impact force, or high energy.
However, if a high-energy-applying device is used from the stage of manufacturing at which the core particles and the fine particles of charge control agent are combined, affixing/film formation of the fine particles proceeds before the fine particles have been uniformly dispersed. As a result, the charge control particles may become affixed to the core particles in a non-uniform state, or a film of non-uniform thickness may be formed. This may lead to manufacturing of electrophotographic toner which lacks charging stability.
Conventional art has also been proposed in which functional fine particles such as charge control agent are dispersed over and attached directly to colored core particles, and then affixed and/or formed into a film thereon. However, no electrophotographic toner has been proposed in which polarity can be controlled even when using the same core particles. Further, no proposal has noted the advantages and effects which could be obtained, in manufacturing electrophotographic toner, by providing a step after production of the core particles, in which they are given a charge of the required polarity.